A cutting tool which uses a cutting insert having a circular-arc-shaped cutting edge is widely used as it is suited for use in three-dimensional machining of shapes. A ball end mill and a radius end mill, etc., are known as this type of cutting tool. A cutting insert having a circular-arc-shaped cutting edge can be used suitably for copy machining, etc., not only for a rotary cutting tool but also for a cutting tool for lathes. The peripheral side surface of such cutting insert having a circular-arc-shaped cutting edge has a curved face which corresponds to the circular arc shape of the cutting edge.
If a plurality of circular-arc-shaped cutting edges is provided in the cutting insert, a part of the peripheral side surface of the cutting insert which comes into contact with the wall surface of an insert seat of a tool body may be a curved face corresponding to the circular arc shape of the cutting edge. In that case, the cutting insert is likely to be fixed unstably and may be shifted in the direction of rotation during cutting. For example, Patent Document 1 proposes a cutting insert and a cutting tool involving a measure for preventing rotation of the cutting insert in the rotating direction. The cutting insert 1 in Patent Document 1 comprises two circular end surfaces and a peripheral side surface and has circular-arc-shaped cutting edges which are formed in an intersecting edge between the peripheral side surface and the two end surfaces. By forming a plurality of flat portions on part of the peripheral side surface and bringing the flat portions into contact with the wall surface of an insert seat, shifting of the cutting insert in the rotating direction is suppressed.
Patent Document 2 describes another example of conventional cutting inserts and cutting tools. The cutting insert in Patent Document 2 comprises a first end surface, a second end surface and a peripheral side surface, and has a plurality of cutting edges formed in an intersecting edge between the first end surface and the peripheral side surface. In an embodiment in Patent Document 2, the cutting insert is formed, for example, in a shape of 120-degree rotational symmetry, i.e., 3-fold rotational symmetry, with respect to the axis passing through the first and second end surfaces. The second end surface serves as a surface to be seated onto an insert seat. A plurality of contact surface portions is formed on the second end surface. In a side view, the cutting edge has a portion which is inclined so as to be closer to the second end surface. In a side view, the contact surface portion has a portion which is inclined in the same direction as the direction of the inclined portion of the cutting edge.